1. Field of the Invention
The present invention relates to a toner image fixing apparatus for an electrostatic transfer type image forming machine such as a copying machine, a printing machine or the like.
2. Description of the Related Art
Conventionally, electrostatic transfer type copying machines have been widely used as one of business machines. In the electrostatic transfer type copying machine, a document is illuminated by a light source, and the light image reflected from the document is focused onto a surface of a cylindrical photoconductive drum by an optical system so as to form the image of the document thereon. When the light image of the document is projected on the surface of the photoconductive drum having, been electrified previously, an electrostatic latent electrostatic latent image is formed thereon corresponding to the light image projected, and thereafter, the electrostatic latent image is developed into a toner image by a developing unit. Then, the toner image is transferred onto a copying paper running at a predetermined transfer speed VP by a transfer unit. The copying paper, onto which the toner image has been transferred, is fed to a fixing unit at a predetermined fixing speed VH in order to fix the toner image thermally, and then, the copying paper is discharged outside of the copying machine.
FIG. 1 is a schematic longitudinal cross sectional view showing a portion of a conventional electrostatic transfer type copying machine 1.
Referring to FIG. 1, after a toner image is transferred onto a copying paper running at the transfer speed VP by the aforementioned transfer unit (not shown), the copying paper is fed to a fixing unit 4 with the aid of a guide member 3. The fixing unit 4 comprises an upper heating roller 5 and a lower heating roller 6. The upper heating roller 5 is constituted by a cylinder made of metal, and is arranged rotatably so that the axial direction thereof is parallel to a horizontal direction. The upper heating roller 5 installs a heating lamp 7 used as a heating source inside thereof, and a coating layer of a resin such as Teflon.RTM. resin is formed on the outer surface of the upper heating roller 5. On the other hand, the lower heating roller 6 is made of a material such as silicon rubber, wherein the material of the lower heating roller 6 has a hardness of 30.degree. or less which is prescribed in the Japanese Industrial Standard (referred to as JIS hereinafter) K6301-1975.
In the conventional copying machine 1, the temperatures HT of the surfaces of the upper and lower heating rollers 5 and 6 (referred to as a fixing temperature HT) are set so as to satisfy the following inequality (1): EQU TT+30.ltoreq.HT.ltoreq.TT+40 [.degree. C] (1)
wherein TT is a melting temperature of the toner to be used as a developer in the copying machine 1. That is, the upper heating roller 5 is heated by the heating lamp 7, and the lower heating roller 6 is pressed on the upper heating roller 5 by a mechanical means (not shown). Therefore, the lower heating roller 6 is heated by the upper heating roller 5, resulting in that the surface temperature of the lower heating roller 6 becomes substantially the same as that of the upper heating roller 5.
Furthermore, the fixing speed VH corresponding to the rotation speeds of the heating rollers 5 and 6 are set so as to satisfy the following inequality (2): EQU 0.99 VP.ltoreq.VH.ltoreq.1.0 VP (2)
wherein VP is the transfer speed, as described above. If the fixing speed VH at which a copying paper 2 is fed by the fixing unit 4 is larger than the aforementioned transfer speed VP, for example, a tensile force is applied to the copying paper 2 while a toner image is transferred thereonto, and the toner image may not be transferred thereonto properly in the case of a considerably large tensile force.
In the conventional electrostatic transfer type copying machine 1, the distance lH between the edge of the aforementioned guide member 3 on the side of the fixing unit 4 and an intersection at which a virtual line 9 extending in a longitudinal direction of the guide member 3 intersects either of the heating rollers 5 and 6 is set so as to satisfy the following inequality (3): EQU 0.1 DH.ltoreq.lH.ltoreq.0.15 DH (3)
wherein DH is a diameter of the upper heating roller 5. If the aforementioned transfer speed VP increases, a relatively large force is applied to the copying paper 2, resulting in that rumples are apt to be easily formed on the copying paper 2. In order to suppress the occurrence of the rumples, it is desirable to strongly squeeze the copying paper 2 by the guide member 3. In order to improve the squeezing effect, the end portion 8 of the aforementioned guide member 3 is positioned relatively near to the upper heating roller 5.
Recently, the aforementioned electrostatic transfer type copying machine 1 has been improved considerably in miniaturizing the size and lightening the weight thereof. Generally speaking, it is recognized experientially that the transfer speed VP becomes slower as the size of the copying machine becomes smaller. Thus, when the transfer speed VP becomes slower, the copying paper 2 is heated excessively if the fixing temperature HT is set to satisfy the inequality (1), as in the conventional copying machine 1. Then, snaking patterns are generated on the copying paper 2 discharged on a discharge tray 10 by a cooperative pressing action between the heating rollers 5 and 6, and also rumples are caused on the copying paper 2 if the snaking patterns becomes seriously large.
The fixing temperature HT is set so as to satisfy the inequality (1) because of the following reasons. In the case that the fixing temperature HT is too low, the toner attached to the copying paper 2, transported to the vicinity of the fixing unit 4, is not melted sufficiently, resulting in a portion of the toner which does not melt adhering to the aforementioned heating roller 5, and then, the remaining toner may adhere to the next copying paper 2 to deteriorate the quality of toner image. It is generally referred to as a low temperature offset phenomenon. On the other hand, in the case when the fixing temperature HT is too high, the toner on the copying paper 2 is melted excessively, resulting in that part of the toner melted excessively adheres to the upper heating roller 5. This generally referred to as a high temperature offset phenomenon.
Furthermore, when the transfer speed decreases as described above, the fixing speed VH set so as to satisfy the inequality (2) also decreases, resulting in the copying paper 2 being heated excessively, as described above.
Furthermore, when the aforementioned distance lH is set so as to satisfy the inequality 3), the end portion 8 of the aforementioned guide member 3 is positioned at a position considerably near to the upper heating roller 5. That is, the copying paper 2 is heated by the fixing unit 4 as described above, however, radiant heat is applied to the copying paper 2 positioned in the vicinity of the guide member 3 from the fixing unit 4. Therefore, the heat to be applied to the copying paper 2 is comprised of not only the transfer heat, which is transferred from the heating rollers 5 and 6, but also the aforementioned radiant heat.
In order to prevent such a state, it has been considered to minimize the contact distance LH between the heating rollers 5 and 6 to be as small as possible. Such a technique can be realized by increasing the distance between the center points of the heating rollers 5 and 6. However, since the conventional lower heating roller 6 is made of a relatively soft silicon rubber material, the pressing force between the heating rollers 5 and 6 becomes weak only when the distance between the heating rollers 5 and 6 is increased, such that the fixing process can not be performed properly.
Furthermore, when the aforementioned fixing speed VH decreases, the copying paper 2 is warped before the fixing unit 4, and, as a result, the running path of the copying paper 2 becomes long while heat is applied to the copying paper 2 from the guide member 3 which is heated by the fixing unit 4, resulting in the copying paper 2 being heated excessively, as described above.
Furthermore, in order to prevent the rumples from occurring in the copying paper 2 as described above, the paper 2 should be strongly squeezed by the guide member 3 so as to make the surface of the copying paper 2 smooth. However, the aforementioned squeezing effect can not be expected since the copying paper 2 is warped, as described above.